This invention relates to a heat reflecting glass plate for use in buildings or vehicles to shut out exterior heat and prevent transfer of interior heat to the exterior, and more particularly to a glass plate having a heat reflecting multilayer coating which includes at least one layer of silver or copper. The heat reflecting glass plate can be used as a component of a laminated glass or an insulated glass.
In conventional heat reflecting glass plates it is prevailing to use a coating film of silver which is high in infrared reflectance. However, a silver film is relatively low in durability and wear resistance and also in transmittance for visible light. Therefore, it is usual to form a multilayer coating by providing a transparent metal or metal oxide film on each side of a silver film.
For example, JA-A No. 62-41740 shows a heat reflecting three-layer coating in which a Ag film overlies a ZnO film and is overlaid with another ZnO film. Further, there are proposals of more intricated multilayer coatings. JP-A No. 59-165001 shows a four-layer coating consisting of an indium-tin oxide (ITO) film deposited on a glass surface, an Ag film on the oxide film, a film of a fully oxidized metal selected from Al, Ti, Ta, Cr, Mn and Zr on the Ag film and another ITO film as the outermost layer. JP-A No. 63-183164 shows a four-layer coating consisting of a transparent Zn-Sn alloy film deposited on a glass surface, a Ag film on the alloy film, a film of a metal selected from Ti, Zr, Cr and Zn-Sn alloy on the Ag film and a TiO.sub.2 film as the outermost layer. JP-UM-A No. 62-37052 shows a four-layer coating consisting of a ZnO or ITO film deposited on a glass surface, a Ag or Cu film on the oxide film, a film of a metal selected from Al, Ti, Ni, Zn, Cr and their alloys on the Ag or Cu film and a ZnO or ITO film as the outermost layer, on condition that at least one of the inner and outer oxide films is insufficient in the degree of oxidation. JP-UM-A No. 62-37051 shows a five-layer coating consisting of a ZnO or ITO film on a glass surface, an Al or Zn film on the oxide film, a Ag or Cu film on the al or Zn film, another Al or Zn film on the Ag or Cu film and another ZnO or ITO film as the outermost layer.
However, these heat reflecting multilayer coatings are still unsatisfactory particularly in moisture resistance. Under high-temperature and high-humidity conditions experienced in summer the silver film in the multilayer coatings is liable to undergo agglomerative oxidation by the influence of moisture to result in appearance of spot-like defects. It is inevitable that the transparent oxide films in the multilayer coating is permeable to moisture to some extent. Even in the case of a multilayer coating including a metal film such as Al or Zn film as a barrier layer, it is likely that the barrier effect diminishes by oxidation of the metal film itself. Of course it is impermissible to make the barrier metal film very thick because of lowering of transmittance of the coating for visible light.